High-resolution spectroscopy of difference and combination bands of SF6 to elucidate the nu(3) + nu(1) - nu(1) and nu(3) + nu(2) - nu(2) hot band structures in the nu(3) region

The strong infrared absorption in the nu(3) S-F stretching region of sulphur hexafluoride (SF6) near 948 cm(-1) makes it a powerful greenhouse gas. Although its present concentration in the atmosphere is very low, it is increasing rapidly, due to industrial pollution. The ground state population of this heavy species is only 32% at room temperature and thus many hot bands are present. Consequently, a reliable remote-sensing spectroscopic detection and monitoring of this species require an accurate modelling of these hot bands. We used two experimental set-ups at the SOLEIL French synchrotron facility to record some difference and combination bands of SF6: (1) a new cryogenic multiple pass cell with 93 m optical path length and regulated at 163 +/- 2 K temperature and (2) the Jet-AILES supersonic expansion set-up. With this, we could obtain high-resolution absorption spectra of the nu(3) - nu(1), nu(3) - nu(2), nu(1) + nu(3) and nu(2) + nu(3) bands at low temperature. These spectra could be assigned and analysed, thanks to the SPVIEW and XTDS computer programs developed in Dijon. We performed two global fits of effective Hamiltonian parameters. The first one is a global fit of the ground state, nu(2), nu(3), nu(3) - nu(2), nu(2) + nu(3), 2 nu(3) and 2 nu(3) - nu(3) rovibrational parameters, using the present spectra and previous infrared, Raman and two-photon absorption data. This allows a consistent refinement of the effective Hamiltonian parameters for all the implied vibrational levels and a new simulation of the 2 nu(3) + nu(2) - nu(2) hot band. The second global fit involves the present nu(3) - nu(1) and nu(1) + nu(3) lines, together with previous nu(1) Raman data, in order to obtain refined nu(1) parameters and also nu(1) + nu(3) parameters in a consistent way. This allows to simulate the nu(3) + nu(1) - nu(1) hot band.